This invention relates generally to the field of cryopumps. Cryopumps condense and adsorb gases on cryopumping surfaces cooled to cryogenic temperatures by a cryogenic refrigerator.
Typically, the cryopumping surfaces include a low temperature array operating in the range of about 4.degree. K. to about 25.degree. K. and a high temperature array operating in the range of about 70.degree. K. to about 130.degree. K. The primary pumping surface is the low temperature array. The high temperature array is positioned between the primary pumping surface and a work chamber to be evacuated and closes a radiation shield which surrounds the low temperature array.
High boiling point gases such as water vapor are condensed on the high temperature array. Lower boiling point gases pass through the high temperature array to the low temperature array wherein they are condensed. The lower temperature array may include an adsorbent such as charcoal or a molecular sieve to remove very low boiling point gases such as hydrogen, helium, and neon. The above condensation and adsorption ensures a high vacuum in the surrounding vessel and at an adjoining processing or work chamber.
Once the high vacuum has been established, work pieces may be moved into and out of the work chamber through partially evacuated load locks. Each time the work chamber is opened, additional gases enter therethrough. These gases are then condensed onto the cryopumping surfaces to evacuate the chamber and provide low pressure for processing. Also, processing gases introduced with the work chamber are condensed onto the cryopumping surfaces.
After several days or weeks of continued processing, the gases that condense and adsorb on the cryopanels begin to saturate the cryopump. It is necessary to release trapped gases by regeneration or defrosting, since the cryopumps are capture pumps and not throughput pumps. During regeneration the cryopump is shut down temporarily so that the cryopumping surfaces warm up and release the trapped gases. The released gases are then purged from the work chamber.
A pressure relief valve is used to avoid dangerous levels of high pressure in the cryopump during regeneration. Typically, the pressure relief valve has a spring-loaded valve held against an O-ring seal which opens when the pressure in the cryopump chamber exceeds about 3 pounds per square inch gauge (PSIG). A filter standpipe may be provided to capture debris (i.e. process debris and particles of charcoal from the adsorber) before it can accumulate on the O-ring seal. The screen filter standpipe is made of porous material which allows the free flow of gas, water, and liquid cryogens therethrough while retaining contaminating debris within the vacuum vessel. If debris were to reach and collect on the O-ring seal, pump-down and start-up would be virtually impossible without cleaning.
To warm up the cryopump during regeneration, a warm gas purge may be performed to decrease warmup time. A warm gas purge warms up both the low temperature array and the high temperature array and ensures that substantially all gases are flushed out of the cryopump. Warming of the cryopump may be supplemented by an electric heater on the refrigerator.
After warmup, the cryopump is rough-pumped to obtain a pressure low enough for cooldown. During the cooldown, all valves are closed so that the cryopumping surfaces can condense or adsorb all residual gases within the cryopump. A high vacuum is obtained by first pumping water, then argon, and nitrogen, etc.